Initialized by the vast increase in information that needs to be processed, optical storage systems have become more and more important particularly because of their high storage density per area which can be more than 10 times as high as that of systems utilizing a magnetic medium. Video disk, digital audio disk, digital information storage disk, etc. are some examples of memory systems employing optical storage media.
In most optical storage systems rotating optical disks are used on which the information is digitally stored in concentric circular tracks in an ordered, predefined manner to allow chronological fast reading and fast random access to desired blocks of data.
For reading, normally an optical head is used to scan the disk, moving disk and head relative to each other. The head focuses a light beam onto the storage surface and detects the reflected portion thereof. The reflected beam is then transformed into an electric output signal.
To reduce reading errors, to improve the signal to noise ratio (S/N ratio), and to avoid optical head damage it is necessary to control the distance and inclination between the head and the optical-storage medium. For this purpose focusing-error and tracking-error detection is required.
For writing, normally the same optical head can be used to emit short pulses of high laser power heating the optical storage medium, e.g. an optical phase change medium to above the melting point, thus leaving marks of different reflectifity.
Two of the main requirements of optical heads, also known as pickup devices, are compact size to allow high speed access, and accurate tracking and focusing to allow high density data storage.
Starting from the earlier systems that employ external light sources, single optical lenses and photodetectors, which are bulky and massive, the trend is towards semiconductor optical heads constructed with integrated optics using waveguides, light- sensitive elements, grating couplers and lasers, thus allowing a reduction of size and weight.
One important step in order to reduce size was the employment of grating couplers. Because they are not as well known as other elements which have been described in numerous articles and books, we have listed references relating to grating couplers and their applications that are representative of the state of the art and which include:
Article "Grating coupler for efficient exitation of optical guided waves in thin films," published in Appl. Phys. Lett., Vol. 16, June 1970, pp. 523-525, gives a funda- mental description of the grating coupler principle.
Article "Waveguide grating lenses for optical couplers," published in Applied Optics, Vol. 23, No. 11, June 1984, pp. 1749-1753 discloses a method of constructing chirped grating couplers using a computer and an electronic beam exposure system.
Article "Aberration characterizations of a focusing grating coupler in an integrated- optic disk pickup device," published in Applied Optics, Vol. 26, No. 22, November 1987, pp. 4777-4782 reports on theoretical calculations of ray and wave aberrations of a focusing grating coupler (FGC).
With respect to tracking and focusing, the following references are the closest prior art known to the applicant:
"An integrated-optic disk pickup device," published in IEEE Journal of Lightwave Technology, Vol. LT-4, No. 7, July 1986, pp. 913-917, is an article where a monolithic- integrated optical-disk pickup device is proposed. This pickup device is constructed with integrated optics and has been realized on a silicon substrate. The pickup device comprises a twin grating focusing beam splitter (TGFBS), a focusing grating coupler (FGC), a wave guide, and several photodiodes (PD) in a film waveguide layer. A laser beam diverging from a butt-coupled external laser diode is focused by the FGC into one point on an optical disk. The reflected wave is collected and split by the TGFBS. Four PD's receive the back-coupled wave, and external electronic devices have to be employed to separate readout, focusing- error, and tracking-error signals.
U.S. patent U.S. Pat. No. 4 760 568, "Optical information processing device," discloses an optical head device similar to the device described in the above cited reference. The inventor has placed the laser into a hollow to reduce stray beams emitted from the laser, which are injected into the photodetectors placed next to the laser.
These known systems are based on silicon-tech- nology which does not allow the integration of a laser. The employment of other semiconductor materials, such as Gallium arsenide (GaAs), allowing the fabrication of optical pickup devices with integrated laser-structures, has not been suggested in the art. Hitherto an external laser had to be fixed to the device.
Some of the problems, caused by the employment of a separate laser are energy loss of the laser beam which has to pass through the interface between laser and device, and stray beams caused by inhomogeneities. In addition, it is necessary to adjust the position of the external laser with high accuracy during pack- aging.
In order to provide high density there is a demand for accurate tracking and focusing, based on tracking-error and focusing-error detection systems. The tracking- and focusing-error detection system of the devices described in the references consists of a focusing grating coupler deflecting the beam emitted from an external laser, out from the device and focusing it to form a single spot used to obtain the readout, tracking-error, and focusing-error signals. The three signals are dependent on each other and a final separation of the signals requires complicated and complex analyser circuitry. In addition to the required complexity, there is the problem that complete decoupling of the signals is not possible without interference. The accuracy of these error detection systems is limited by these interferences.
Despite the fact that the devices known to the applicant still suffer from a number of deficiencies, no suggestion has yet been made to make use of compound semiconductor materials, as GaAs for example, allowing the integration of all optical elements. Furthermore the employment of waveguide networks and optical and electrical separation of the error detection systems has not been shown before.